| Abstract: | Total oestradiol concentrations in the serum of young female rats were high and decreased after about 21 days of age. High affinity serum oestradiol-binding components (EBP), however, fell steadily from 5 to 23 days of age while the unbound oestradiol-17 beta fraction, which was low early in development, increased between 21 and 28 days of age. Injection (i.v.) of immature (EBP-rich) oestradiol-free serum into 21-day-old female rats led to a decrease in the unbound oestradiol fraction and an increase in serum FSH concentrations. Incubation of uterine tissue with 3H]oestradiol, with or without the addition of diethylstilboestrol (DES), showed the rate and degree of total and DES-suppressible uptake of 3H]oestradiol to be greatest from buffer, less from adult (EBP-poor) serum and negligible from immature (EBP-rich) serum; moreover, there was a positive correlation between the degree of uptake of 3H]oestradiol and the unbound fraction of 3H]oestradiol in the incubate. It is concluded that, at least in young rats, oestradiol activity depends more on the availability of free oestradiol than on its total plasma concentrations. |
